1. Field of the Invention
The present invention relates to a linear actuator utilized as a drive source of various devices including a car and more particularly to a linear actuator for accumulating one-way motion from an alternating motion with a micro amplitude by an alternating drive source for generating drive force back and forth in the axial direction, thereby creating a direct motion.
2. Prior Art
Devices such as an automatic transmission (automatic MT) for driving a shift lever for switching the combination of the shift gears for a car by a motor instead of a hand and a variable operating valve device for controlling optimally the lift amount of the valve of an engine or the opening period thereof according to the operation state realize the compatibility of improvement of the comfortableness of a car with improvement of fuel expenses, thus future enlargement of use is expected. Unlike other more general machines driven directly by a rotation actuator such as a motor for repeating continuously a predetermined operation, the concerned devices are controlled directly at optimum positions according to the circumstances and instead of the rotation actuator, are driven by a linear actuator for outputting a direct motion.
Therefore, in the devices such as the automatic transmission and variable operating valve device, the performance of the linear actuator controls the performance of the overall devices. For example, if there is a linear actuator for responding at a high speed available, a device using it can function at a high speed and if there is a compact linear actuator with high thrust available, an overall device incorporating it can be made compact. To minimize the control delay for a control subject such as the transmission gear ratio or valve operation for sudden changes in the operation conditions of a car such as the car speed and engine load changing every moment and maintain always the car in an optimum state, particularly the characteristic of the high speed response aforementioned is important.
As a linear actuator, conventionally, an article structured so as to create a direct motion by a ball screw and a worm gear from the rotary motion of the motor has been used, though to generate large thrust of the direct motion with limited motor torque, a concept is used that the motor is rotated at a high speed to ensure the output, and it is decelerated at the stage of realization of direct motion, thus large thrust is generated. Therefore, to speed up the direct motion, it is essential to speed up the rotation of the motor. On the other hand, the speedup of the linear actuator is evaluated by the shortness of the time required to move by a predetermined stroke from the standstill. This indicates that in a conventional linear actuator driven by a motor, the rotation system must be put into a high-speed rotation state in a short time, so that the output of the motor is consumed to increase the kinetic energy of the rotation system. In other words, it may be said that at start time of the linear actuator, a large inertia resistance is acted. In the linear actuator of the conventional type, the inertia resistance may be considered to be an obstacle to speedup.
As a means for avoiding the obstacle to speedup, a linear actuator for accumulating one-way motion from an alternating motion with a micro amplitude by an alternating drive source for generating drive force back and forth in the axial direction of a magnetostrictor, thereby creating a direct motion is known. Generally, the alternating drive source obtains only a micro amplitude, though can generate large thrust, so that there is no need to permit the magnetostrictor to move back and forth at a high speed, then decelerate, thereby enlarge the thrust, thus it can be expected that the inertia resistance will be reduced.
As a prior art of the linear actuator by the alternating drive source using the magnetostrictor, for example, as described in Patent Document 1, a linear actuator for acting a fluid pressure on a piston using a check valve, thereby generating a direct motion is proposed. It is disclosed as a one that a hydraulic chamber is formed at the end of the member for moving back and forth with a micro amplitude by the magnetostrictor, and a fluid is pumped in one direction by two check valves functioning as an inlet valve and a discharge valve, and the fluid pressure is acted on the piston, thus direct drive force is generated.
Further, as another prior art, as described in Patent Document 2, a linear actuator for generating a direct motion using a wedge mechanism for permitting only a relative motion in one axial direction is proposed. Also in this case, a linear actuator is disclosed that a first wedge mechanism having an alternating motion member for moving back and forth with a micro amplitude by the magnetostrictor for permitting only a relative motion in one axial direction is arranged between the alternating motion member and the output member for executing the direct motion, and furthermore, a second wedge mechanism for permitting only a relative motion in one axial direction is arranged between the output member and the fixed frame member, and the one-direction motion of the alternating motion is accumulated by a combination of the wedge mechanisms, thus a direct motion is generated.
Patent Document 1: Japanese Patent Application Laid-open Publication No. Hei 9 (1997)-303451
Patent Document 2: Japanese Patent Application Laid-open Publication No. 2005-33978